Compensatory Growth Responses Among Species And Functional Groups In Relation To Community Temporal Stability

Stability is an effective index on evaluating ecosystem function,which is affected by three factors:species diversity,interspecific interactions and patterns of population fluctuation.Global change and human disturbance can change not only the original state and temporary stability of a community,but also the mechanism of maintaining the temporary stability of the community.Compensatory growth is a response and ecological adaptation strategy for plants to compensate for the loss of tissue after grazing or picking damage.After the disturbance,the different species and functional groups composed of similar species in the community will have different compensation response dynamics due to their different response degree and way,and the different compensation model generated within or between species will play a stabilizing role in the whole community.However,a large number of studies,more attention has been paid to the compensatory growth of the community as a whole,or simply verified the mechanism of maintaining the stability of the community.In view of this,a long-term field control experiment,treated with simulated grazing(including three level of no clipping,moderate clipping and heavy clipping)and atmospheric nitrogen deposition(including two level of no fertilization and fertilization)was conducted in an alpine meadow on the Qinghai Tibet Plateau.And based on the species diversity(SR,D and H')for many years,the aboveground net primary productivity of community,species and functional groups(2009-2019)to reveal the response of the ecosystem to the changing environment.The main mechanism of maintaining the stability of productivity under different disturbance factors and disturbance intensity,and the relative importance of the factors influencing the stability were evaluated or detected based on the compensation response model of species and functional groups[over compensation(OC),equal compensation(FC)and under compensation(UC)].In this study,The following specific research contents were included:(1)the response of community species diversity to long-term simulated grazing and nitrogen deposition;(2)the influence of simulated grazing and atmospheric nitrogen deposition on the temporal stability of the net primary productivity and productivity of the community;(3)the dependence of diversity-productivity,diversity-stability and productivity-stability on disturbance factors and disturbance intensity;(4)the response of compensatory growth ability and models of community,species and functional groups to simulated grazing and nitrogen deposition;(5)the coupling relationship between compensatory response dynamics of species and functional groups and community temporary stability.In this study,the effects of clipping,fertilizating and their interaction on the above community characteristics were mainly investigated,but the time response of the characteristics was not concerned.Based on the annual average value of each measurement index,the main effect and interaction effect of the disturbance factors were determined by using the mixed effect ANOVA model,and the relationship among the characteristics indexes was determined by regression analysis.The compensatory growth model was determined by a single sample t-test,and the factors affecting the temporal stability of productivity and their relative contributions were revealed based on stepwise regression analysis.The specific research results are as follows:(1)SR,D and H' increased after clipping and decreased after fertilizing.clipping and fertilizing showed counter effects on SD,which was consistent with previous research results.However,we found that the effect of fertilization(FSR=433.29,FD=130.91 and FH=524.19)was much greater than that of clipping(FSR?19.38,FD=17.46 and FH'=2.11),which should be related to the fact that alpine meadow is a typical nitrogen limitation system.There was no significant effect of clipping on H',but it was related to the change of H' in the clipping gradient under the condition of fertilizing and non-fertilizing.After fertilizing,D decreased only under two clipping treatments,while H' significantly decreased in all clipping level,indicating that the loss of species caused by fertilizing was mainly due to the decrease of the number and relative abundance of rare species.(2)Both clipping and fertilization increased the net primary productivity(ANPPcom),and the effect of fertilization(F(1,3)=208.55)was greater than that of clipping(F(2,2)=9.98).The interaction of clipping and fertilization had no significant effect on ANPPcom,that is,whether there was clipping or not,fertilization could always significantly increase ANPPcom,and the highest ANPPcom occurred under moderate clipping.This is consistent with the grazing optimization hypothesis and compensatory continuum hypothesis of compensation growth theory.(3)Clipping and fertilization had significant effects on the temporary stability(Scom)of ANPPcom.heavy clipping reduced Scom,fertilizing increased Scom,and clipping(F(2,2)=154.71)had greater effects than fertilization(F(1,3)?18.71),but the interaction between clipping andfertilizing had no significant interaction on Scom.(4)As clipping and fertilization have significantly opposite effects on species diversity and both significantly improve ANPPcom,fertilization results in a significant negative correlation between SR,D and H' and ANPPcom on the compound community gradients including all treatment combinations and on the different clipping level communities on the fertilization gradients.Because clipping has no significant effect on H',SR,D and H' are positively correlated with ANPPcom in fertilized or non fertilized communities,except for H' and ANPPcom in fertilized communities.(5)The relationship between Scom and SR,D,and H' was negative or no correlation in most of the treatment compound communities,but no positive correlation,which was related to the opposite effects of clipping and fertilizing on diversity and Scom,The negative correlation between SR,D and H'and Scom in the compound community gradients including all treatments and the negative correlation in the moderate and heavy clipping communities in the fertilization gradients all reflected the effect of clipping and fertilizing.In no clipping communities,there was no correlation between diversity and the temporal stability in fertilization gradient because fertilization decreased diversity and increased stability at the same time.Similarly,on the clipping gradients,there is no correlation between Scom-SR and fertilization community,negative correlation between Scom-H' and Scom-D and no correlation between them all reflect the strong influence of clipping disturbance on diversity stability relationship.(6)There was no significant correlation between ANPPcom and Scom on the gradient of compound community,fertilization or clipping level,which indicated that the change of ANPPcom was not the cause of Scom change,ANPP and its variability were equal and synchronous,which led to relatively constant Scom.(7)The compensatory growth ability and pattern of community level were affected by clipping and fertilization.Compared with the community without clipping and fertilizing,the moderate clipping community is full compensation growth model regardless of fertilization,while the heavy clipping community is full compensation growth model without fertilization and over compensation growth model when fertilization.(8)The compensatory growth model at the species level was diversified under different treatment conditions.Although 63.3%of the species in the moderate and 56.6%of the species in the heavy clipping communities were over compensated,fertilization caused more species in the heavy clipping communities to improve their compensatory growth ability.The effect of fertilization on the compensatory growth ability of species is related to the nature of species,which can lead to the loss of nitrogen sensitive species(mainly Leguminosae),and promote the regeneration and reproduction of species with vegetative storage organs.(9)The compensatory growth ability and pattern of plant functional groups(PFGs)are also complex,and vary with experimental treatment.The Grasses functional group showed a under compensatory growth model in heavy clipping and non fertilization communities,however,in other treatment showed an full compensatory growth model,so it had a medium and low compensatory capacity;the Leguminosae functional group showed an over compensatory growth model in the moderate clipping and non fertilization communities,and an full compensatory growth model in the other treatment,with a medium and high compensatory capacity;The functions of Cyperaceae and Compositae are in the same compensatory growth pattern.In addition to the above plants,the functional groups of Forbs have higher compensatory growth ability,showing over compensatory growth mode in heavy clipping and fertilizing,andfull compensatory growth model in other treatment.(10)After the disturbance of clipping and fertilizing,the compensation response dynamics among different species and functional groups have an important impact on Scom,and reflect the change of the main mechanism of maintaining stability.Analyses of the summed variance the summed covariance(?Cov),the community SR,species level(spe)and functional group level(pfg)and Scom,showed that ?Covpfg and?Covspe are the two most effective factors to predict Scom,indicating that the asynchronous effect produced by the compensation response of the difference of different species and functional groups determines the temporary stability of the community.However,this kind of effective predictor is disturbance dependent.The most effective predictors of moderate and heavy clipping communities on the fertilization gradient are ?Varspe and SR respectively,that is,the portfolio effect and species number caused by the independent fluctuation of species are the main mechanisms to maintain stability;in the fertilization communities on the clipping gradient,?Covpfg is the factor that contributes the most to Scom,and The main mechanism is the asynchronous effect between functional groups.In non clipping and non fertilization communities,alternative predictors(SR,?Varspe,?Covspe,?Varpfg and?Covpfg)can not explain the mechamism of stability maintenance well.In conclusion,clipping and fertilization,as the two most common interference modes in alpine meadow,have complex and different impacts on community characteristics.Moderate clipping can significantly increase species diversity and community temporary stability.Fertilization can significantly reduce species diversity,but also increase ANPPcom and Scom.This is different from the results of many previous studies on ecosystem based on equilibrium,which show that the relationship between diversity and ecosystem function(including productivity and its temporary stability)is often negative and disturbance dependent.Scom is determined by the compensation response relationship between species and functional groups after disturbance of clipping and fertilization.The asynchronous effect from these two level is the main mechanism to maintain the temporary stability of alpine meadow productivity in heterogeneous environment.At the same time,our research also emphasizes that,because the strong negative effect of fertilization on species diversity is far greater than the positive effect of clipping,under the background of the increasing global atmospheric nitrogen deposition,the species diversity of alpine meadow may be further lost.In the grassland management and improvement,the intensity of grazing or clipping can be reasonably controlled,the amount of fertilization can be effectively reduced,and clean technology can be developed to reduce atmospheric nitrogen deposition and it is the fundamental measure to effectively alleviate the decline of species diversity.